Patent GB-941736-A (GB)
Title: Improvements in or relating to hydrostatic transmissions
IPC: F16H61/46
Filed: 1961-06-14
Abstract: 941,736. Motor vehicle driving gear. R. C. PEARSON. March 14, 1962 [June 14, 1961], No. 21562/61. Heading B7H. [Also in Division F2] In an hydrostatic transmission for a motor vehicle, especially an endless track vehicle, in which power from a prime mover is transmitted to the tracks or road wheels on opposite sides and the vehicle may. be steered by changing the motion of the tracks or wheels, a two position valve is fitted between the main supply and return lines between the pump and motor(s), such that in normal position of the valve it is closed and no interflow between the lines occurs but in its other position the valve is open to enable fluid to flow freely through the valve from one line to the other, and means are further provided to open such valve in an emergency or when desired. The vehicle may be equipped with a pair of motors and the hydraulic system may be as described in Specification 941,735 shown in Fig. 1, an engine 1 drives a variable displacement pump 2, servo pump 3 and booster pump 4, the pumps 3 and 4 drawing fluid from a reservoir 11 through a line 12. The opposite sides of variable displacement motors 21, 22, which drive through bevels 25, 26 the driving sprockets 27 of an endless track vehicle, are connected to lines 15 and 16 from the pump 2, make-up liquid being supplied from the pump 4 through check valves 17 and 18. The pump 3 supplies the servo system through a line 13 and the output of both pumps 3 and 4 are subject to control by pressure relief valves 51. Lines 15 and 16 are connected with the reservoir 11 through pressure relief valves 19 and 20. The pump 2 is operated by a servo and follow up valve 5 connected to link 6 acted upon by springs 7 and 8 which give " feel " and bias the link to a mid-position, the link 6 being connected by a member 9 to a control lever 10. The motors 21 and 22 are controlled by rotating the housings by jacks 30 and 31 subjected to the pressure of springs 35 and to fluid pressure admitted from a control valve 45 through a line pressure change over valve 50, valve 45 also being connected by a line 14 to reservoir 11. A shuttle valve 41 allows pressure fluid from whichever of line 15 or 16 is at higher pressure to supply liquid to a conduit 42 and thence through a shuttle valve 60 to the valve 45. Valve 60 is also supplied from pump 3 at a predetermined pressure so that a minimum pressure is always available at the valve 45, the spool 44 of which is acted upon by springs 63 and 64 and the end chambers of which are connected to the adjacent chambers by passages through the spool 44. At the central position of spool 44, pressure fluid from the line 62 is admitted to both jacks, the predetermined minimum pressure displacing both pistons 34 until a balance with springs 35 is obtained. When spool 44 is moved in one direction or the other, flow of fluid to the corresponding jack is restricted and that jack is subsequently connected to reservoir. The difference in pressure acts on the ends of spool 44 and the spring 63 or 64 through which the initial movement of the spool was effected becomes compressed and the spool returns to the position where there is no supply to the affected jack, and neither is there any leak-off, so that the displacement of this jack is according to the distance moved of the spool 44. The springs 63 and 64 through which this spool is operated provide steering " feel." The jacks 30 and 31 act as pressure responsive means regulating their respective motors automatically above a predetermined pressure, but either jack may be independently operated to reduce the speed of, or reverse the associated motor. Each jack comprises a piston 34 having an aperture 36 and a spring 35, a dynamic balance being achieved between the pressures on opposite sides of the piston, and the orifice 36 damping any tendency to sudden movements. A by-pass valve 100 is connected by the lines 105, 107 to the lines 15 and 16, by the line 111 to the higher pressure line through shuttle valve 41 and is subjected to servo pressure through a line 109 and valve 108, the latter being shown in its normal operating position and having lines 121 connected through line 14 to reservoir. A rod 113 is attached to the spool of valve 108 to reduce the effective area on one side of land 103. With servo pressure available, the spool is in the position shown, but if its supply stops, the pressures on the lands 101, 102 and 103 cause the spool to move to the left and so put the lines 15 and 16 into communication with one another. By operating valve 108 manually, the supply of fluid through line 109 may be cut off and again the lines 15 and 16 are short circuited. The areas of the lands of valve 100 may also be selected so that the valve acts as an overload relief valve, and the valves 19 and 20 may be disposed with. Instead of the enlargement 113, a spring may be provided, the chamber containing which is connected to reservoir. Specification 902,185 also is referred to.